Revert of [XMLHttpRequest] Stop throwing for network error in async mode

LayoutTests/http/tests/xmlhttprequest/resources/cross-origin-unsupported-url.js

 if (self.importScripts)
     importScripts("/js-test-resources/js-test.js");
 
 self.jsTestIsAsync = true;
 
 description("Test cross-origin XHRs to CORS-unsupported protocol schemes in the URL.");
 
 var xhr;
 var errorEvent;
 function issueRequest(url, contentType)
 {
     xhr = new XMLHttpRequest();
     xhr.open('POST', url);
     xhr.onerror = function (a) {
         errorEvent = a;
         shouldBeEqualToString("errorEvent.type", "error");
         setTimeout(runTest, 0);
     };
     // Assumed a Content-Type that turns it into a non-simple CORS request.
     if (contentType)
         xhr.setRequestHeader('Content-Type', contentType);
 
-    shouldNotThrow('xhr.send()');
+    if (self.importScripts) {
+        // Initiating the load on the main thread is not performed synchronously,
+        // so send() will not have an exception code set by the time it
+        // completes in the Worker case. Hence, no exception will be
+        // thrown by the operation.
+        shouldNotThrow('xhr.send()');
+    } else {
+        // The implementation of send() throws an exception if an
+        // exception code has been set, regardless of the sync flag.
+        // The spec restricts this to sync only, but as error progress
+        // events provide no actionable information, it is more helpful
+        // to the user to not follow spec.
+        //
+        // As the initiation of the request happens synchronously in send(),
+        // and it is determined that it is to an unsupported CORS URL, an
+        // exception is expected to be thrown.
+        shouldThrow('xhr.send()');
+    }
 }
 
 var withContentType = true;
 var tests = [ 'http://localhost:1291a/',
               'ftp://127.0.0.1',
               'localhost:8080/',
               'tel:1234' ];
 
 function runTest()
 {
     if (!tests.length && withContentType) {
         finishJSTest();
         return;
     }
     withContentType = !withContentType;
     if (!withContentType)
         issueRequest(tests[0]);
     else
         issueRequest(tests.shift(), 'application/json');
 }
 runTest();